1. Field of the Invention
The present invention relates to discharge tube drive circuits for controlling emission of cold cathode discharge tubes such as fluorescent lamps, and more particularly to discharge tube drive circuits that employ a plurality of drive transformers for driving a plurality of cold cathode discharge tubes.
2. Description of the Related Art
As well known, cold cathode discharge tubes such as fluorescent lamps emit lights by being driven with high frequency drive voltages generated in an inverter. A cold cathode discharge tube of this type is used for lighting purpose, and is also used for a backlight source of a LCD panel, recently. For this purpose, drive transformers are provided at output side of switching circuit included in a discharge tube drive circuit, and cold cathode discharge tubes are connected to output terminals of a secondary coil side in the drive transformers by way of connectors.
Particularly, in case of using cold cathode discharge tubes for backlight of a LCD panel, a plurality of cold cathode discharge tubes is employed, and the cold cathode discharge tubes must emit uniformly.
It has been already known to uniformly control the currents flowing through a plurality of cold cathode discharge tubes by connecting balance transformers to a low voltage side of the cold cathode discharge tubes or by connecting balance transformers to a high voltage side of the cold cathode discharge tubes.
Further, a voltage across electrodes of a cold cathode discharge tube becomes uneven due to unevenness of impedance values of a plurality of cold cathode discharge tubes. Therefore, a current flowing through each cold cathode discharge tube becomes different value, and luminosity of emitting cold cathode discharge tubes becomes different. Accordingly, in case of using cold cathode discharge tubes for backlight of the LCD panel, unevenness of luminosity in a LCD panel occurs, so that it is necessary to control current flowing through the cold cathode discharge tubes to be uniform.
As mentioned above, it has been already introduced technology in manufacturers to uniformly control the currents flowing through a plurality of cold cathode discharge tubes by connecting balance transformers to a low voltage side of the cold cathode discharge tubes, or by connecting balance transformers to a high voltage side of the cold cathode discharge tubes. Due to unevenness of impedance values of the discharge tubes or due to unevenness of stray capacitance between an LCD panel and the cold cathode discharge tubes, even the same drive voltage is applied to all cold cathode discharge tubes, the currents flowing through each cold cathode discharge tube do not become the same. In an LCD-TV, a screen size of the LCG panel has been larger, so that, a plurality of cold cathode discharge tubes is required per one LCD panel. Accordingly, unevenness of luminosity in the LCD-TV tends to occur by the differences of the amount of the current flowing through each cold cathode discharge tube, so that it is essential to adjust the current flowing through each cold cathode discharge tube to be the same.
Conventionally, it is proposed to connect a balance transformer to a low voltage side and/or a high voltage side of a cold cathode discharge tube. However, in this case, it requires (N−1) balance transformers relative to N cold cathode discharge tubes, or it requires a plurality of coils on a magnetic path of a balance transformer corresponding to the number of the cold cathode discharge tubes such as disclosed in Japanese Patent Laid-Open No. 2003-31383 and U.S. Pat. No. 6,781,325. However, if (N−1) balance transformers are employed relative to N cold cathode discharge tubes, these balance transformers occupy a large space and a circuit board becomes larger. Further in such a balance transformer including coils corresponding the number of the cold cathode discharge tubes in one magnetic path, there is a problem where the size of the balance transformer itself becomes larger.
Further in PCT International Publication No. WO2005/038828, primary coils of balance transformers are connected to cold cathode discharge tubes, respectively, and each secondary coil of each balance transformer is configured to be a circuit forming a closed loop. Further in the above PCT publication, it is disclosed that a plurality of cold cathode discharge tubes is connected in parallel to outputs of the drive transformers, and when one of the cold cathode discharge tubes is not activated, the balance transformers work to boost the voltage of the portion.
However, once balance transformers are provided at secondary coil side of a drive transformer, it is necessary to consider insulation since the secondary coils generate high voltage, so that it is also necessary to consider layouts of components upon circuit board design. In addition, the same number or a half number of the balance transformers with compared to the number of the cold cathode discharge tubes are to be used, so that these balance transformers occupied a large area on the circuit board. Published Japanese translation of PCT International Publication for patent application No. 2004-506294 also discloses a similar drive circuit.